825 resultados para 172-1055C
Resumo:
The interval of time represented by marine isotope stages 11 and 12 (~360-470 ka) contains what may be the most extreme glacial and interglacial climate conditions of the Late Pleistocene. It has been suggested that sea level rose by ~160 m at the termination of glacial stage 12. This is 30% greater than the sea level rise that followed the most recent glacial maximum. There have been few detailed studies of the unique conditions that existed during the stage 11-12 time period because of the lack of high-quality core material. This problem has been addressed by the collection of high deposition rate cores from sediment drifts in the western North Atlantic during Ocean Drilling Project Leg 172. Benthic foraminiferal d13C data from cores collected between ~4600 and 1800 m were used to reconstruct bathymetric gradients in deep and intermediate water properties for selected time slices during this glacial-interglacial cycle. During glacial stage 12, the deep western North Atlantic was filled by a water mass that was more nutrient-enriched than modern Antarctic Bottom Water. Above 2000 m, a more nutrient-depleted water mass existed during this glacial stage. Such an intermediate water mass has been described for more recent glacial periods and presumably forms in a more proximate region of the North Atlantic. Interglacial stage 11 water mass properties closely resemble those of the present-day western North Atlantic. A nutrient-depleted water mass (d13C of 0.75-1.0 per mil), similar to modern North Atlantic Deep Water existed between 3500 and 2000 m. This was underlain by a water mass with lower d13C values (<0.75 per mil) that probably was derived from a southern source. Using Leg 172 data, along with previously published results from the Atlantic and Pacific oceans, we estimate a mean global d13C change of 0.95 per mil from stage 12 to stage 11. This is twice the whole ocean ?13C change reported for the transition from the last glacial maximum to the Holocene.
Resumo:
The strength and geometry of the Atlantic meridional overturning circulation is tightly coupled to climate on glacial-interglacial and millennial timescales, but has proved difficult to reconstruct, particularly for the Last Glacial Maximum. Today, the return flow from the northern North Atlantic to lower latitudes associated with the Atlantic meridional overturning circulation reaches down to approximately 4,000 m. In contrast, during the Last Glacial Maximum this return flow is thought to have occurred primarily at shallower depths. Measurements of sedimentary 231Pa/230Th have been used to reconstruct the strength of circulation in the North Atlantic Ocean, but the effects of biogenic silica on 231Pa/230Th-based estimates remain controversial. Here we use measurements of 231Pa/230Th ratios and biogenic silica in Holocene-aged Atlantic sediments and simulations with a two-dimensional scavenging model to demonstrate that the geometry and strength of the Atlantic meridional overturning circulation are the primary controls of 231Pa/230Th ratios in modern Atlantic sediments. For the glacial maximum, a simulation of Atlantic overturning with a shallow, but vigorous circulation and bulk water transport at around 2,000 m depth best matched observed glacial Atlantic 231Pa/230Th values. We estimate that the transport of intermediate water during the Last Glacial Maximum was at least as strong as deep water transport today.
